GTS 10 - changing from Plant to Cross-plant-based preference

We have had many roll-outs of GTS globally, and in the last few roll-outs, we have successfully used the cross-plant based preference model. There are many advantages for us with the cross-plant model, especially where we have multiple plants in each country which are best grouped together. We would now like to change over to the cross-plant model for organisations where the plant model has been previously implemented and used for one or two years. We have a situation where we currently have a 1:1 relationship between admin unit and plant and now want to have 1 admin unit containing a single plant group containing a number of plants. However, we need to preserve the following historical information: - LTVD's issued to customers on the old admin unit - preference records at the plant level for the old admin unit. For the LTVD's I am able to change the admin unit in /SAPSLL/PREVD and the LTVD is then visible linked to the new admin unit. However, it is not so simple for the preference records as these may exist in more than one plant. Does anyone have any experince with changing the system in this way? Are there any other considerations?